1. Field of the Invention
This invention relates to a laser beam printer for reproducing image data on a recording paper by projecting on a photosensitive member a laser beam modulated in accordance with the image data and more particularly to a laser beam printer provided with a controlling device for controlling the timing of emission of the image data.
2. Description of Related Art
A laser beam printer reproduces on a recording paper an image conforming to given image data by projecting in a scanning pattern a laser beam modulated in accordance with the image data on the periphery of a photosensitive drum in which is rotating in the direction along the axis of rotation of the photosensitive drum thereby forming a corresponding latent image on the periphery of the photosensitive drum and subsequently transferring the latent image in the form of a sensible image to the recording paper.
In order that the leading data of the successive scanning lines impinging on the photosensitive drum may be aligned at a fixed position in the circumferential direction of the photosensitive drum and, consequently, the position for inscription of a latent image on the photosensitive drum may be fixed, the emission of given image data in each of the scanning lines is started after the elapse of a prescribed time following the detection of a laser beam at a point preceeding the position for starting the inscription. This detection of the laser beam is accomplished by a SOS (start of scanning) sensor formed of a photodiode, for example, which is disposed at a location preceding the aforementioned position for starting the inscription. U.S. Pat. No. 4,264,120 discloses detection unit which is provided with a photoelectric conversion element for controlling the timing of starting the emission of light data onto the photosensitive drum. This detection unit issues a start signal after the elapse of a prescribed time following the detection of the laser beam.
The SOS sensor issues an output signal on being traversed by the laser beam and, with the elapse of a prescribed time after the output signal surpasses a prescribed threshold, sends forth the leading data of a successive scanning line. The output level of this sensor, however, is susceptibile of variation with numerous environmental conditions. A laser diode which generates a laser beam modulated in accordance with given image data, for example, undergo variation in the amount of generated light with a change in temperature. The output level of the sensor, therefore fluctuates proportionately to this variation. Further, there is a circuit element which monitors the amount of light generated by the aforementioned laser diode and stabilizes the amount of light. The operation of this circuit element may also vary with temperature. Moreover, the aforementioned sensor has the sensitivity thereof affected by a change in temperature and, consequently, the output thereof fluctuates proportionately.
Such a change in the output level of the aforementioned sensor entails the disadvantage that this change reflects itself in the time of detection serving as the standard for the timing of inscription of the leading data of each scanning line and, as a result, induces the phenomonon of jitter, i.e. a deviation of the position for starting inscription of each of the scanning lines projected on the photosensitive drum.
To preclude the occurrence of any undesirable phenomenon of this sort, it has been proposed, as disclosed in Japanese Patent Application Laid-Open 58-127,108, to have two SOS sensors disposed as arranged in the direction of scanning route at the points preceding the position for starting inscription of the leading data and enable the timing of inscription to be controlled on the basis of the time at which the trailing edge of the output of one of the sensors and the trailing edge of the output of the other sensor overlap. In this case, the time of this overlapping is substantially fixed even when the output levels of the two sensors vary. The use of two such sensors not only adds to complication of the control circuit but also requires an adjustment an, not necessarily easy in setting the mutual positions of the two sensors with high accuracy.
When a special sensor having these two sensors integrally incorporated therein is used, the cost of sensor is high.
An object of this invention is to provide a laser beam printer which, by the use of one sensor, enables the position for starting inscription of each of scanning lines to be fixed in spite of a possible fluctuation of the output level of the sensor.
Another object of this invention is to provide a laser beam printer which, by the use of a sensor disposed at a point preceding the position for starting inscription, enables the timing of starting the inscription to be set on the basis of the intermediate time between the rising time and the falling time of an output signal generated by the sensor on detecting the laser beam.
In accordance with the present invention there is provided a laser beam printer capable of modulating a laser beam scanning the surface of a photosensitive member with image data, comprising detection means for detecting the scanning laser beam at a prescribed position and issuing a detection signal which rises when said laser beam reaches said prescribed position and falls when said laser beam passes said prescribed position, first pulse train generating means for generating a first pulse train in response to said rise of said detection signal, second pulse train generating means for generating a second pulse train in response to said fall of said detection signal, counting means for counting the total number of pulses of said first pulse train and said second pulse train, and permit signal output means for issuing a permit signal indicating the timing of supplying said image data when the total number of pulses counted by said counting means reaches a prescribed value.